Three X-15s made 199 flights during a research program which lasted from 1960 through 1968. It was a daring, yet highly successful program that resulted in hundreds of technical reports. It made contributions to the NASA space program of the 1960s and also on the design and flight of the Space Shuttle many years later.

An unofficial motto of flight research of the 1940s and 1950s was "higher and faster." By the late 1950s the last frontier of that goal was hypersonic flight (Mach 5+) to the edge of space. It would require a huge leap in aeronautical technology, life support systems and flight planning. The North American X-15 rocket plane was built to meet that challenge. It was designed to fly at speeds up to Mach 6, and altitudes up to 250,000 ft. The aircraft went on to reach a maximum speed of Mach 6.7 and a maximum altitude of 354,200 ft. Looking at it another way, Mach 6 is about one mile per second, and flight above 264,000 ft. qualifies an Air Force pilot for astronaut wings.

The plane was air launched by NASA's converted B-52 at 45,000 feet and a speed of 500 mph. Generally there were two types of flight profiles: high-speed, or high-altitude. High-speed flights were usually done below an altitude of 100,000 feet and flown as a conventional airplane using aerodynamic controls. High-altitude flights began with a steep, full-power climb to leave the atmosphere, followed by up to two minutes of "coasting up" to the peak altitude after the engine was shut down. "Weightless" flight would last for 2 - 5 minutes as it made a ballistic arc before reentering the atmosphere. A reaction control system was used to maintain attitude above the atmosphere. The reaction controls employed hydrogen peroxide thrusters located on the nose and wings.

Depending on the mission, the rocket engine provided thrust for the first 80 to 120 seconds of flight. The remainder of the normal 8- to 12-minute flight was without power and ended in a 200-mph glide landing. Because the nose landing wheel lacked steering and the main landing gear employed skids, the X-15 had to land on a dry lakebed. The Rogers Dry Lake adjacent to Edwards and Dryden was the intended landing location for all flights, but there were numerous emergency lakebeds selected in advance for emergency landings.

The X-15 program made many accomplishments, here is list of some of its contributions to space flight:

• First application of hypersonic theory and wind tunnel work to an actual flight vehicle.
• First use of reaction controls for attitude control in space.
• First reusable super alloy structure capable of withstanding the temperatures and thermal gradients of hypersonic reentry.
• Development of [a servo-actuated ball] nose flow direction sensor for operation over an extreme range of dynamic pressure and a stagnation air temperature of 1,900 degrees Fahrenheit [for accurate measurement of air speed and flow angle at supersonic and hypersonic speeds].
• Development of the first practical full pressure suit for pilot protection in space.
• Development of inertial flight data systems capable of functioning in a high dynamic pressure and space environment.
• Discovery that hypersonic boundary layer flow is turbulent and not laminar.
• Discovery that turbulent heating rates are significantly lower than had been predicted by theory.
• First direct measurement of hypersonic skin friction and discovery that skin friction is lower than had been predicted.
• Discovery of hot spots generated by surface irregularities. [These last 4 discoveries led to improved design tools for future hypersonic vehicles, including the Space Shuttle.]
• Discovery of methods to correlate base drag measurements with tunnel test results so as to correct wind tunnel data [and thereby improve design criteria for future air- and spacecraft].
• First application of energy-management techniques [for the positioning of the vehicle for landing; these were essential for the landing of the Space Shuttle and all future reusable launch vehicles following their reentry from space.]
• Use of the three X-15 aircraft as testbeds carrying a wide variety of experimental packages.

More intangibly but no less importantly, in the words of the distinguished Langley aeronautical researcher John Becker, who had been an early advocate of the X-15 program, the X-15 project led to "the acquisition of new manned [today, we would say piloted] aerospace flight 'know how' by many teams in government and industry. They had to learn to work together, face up to unprecedented problems, develop solutions, and make this first manned aerospace project work. These teams were an important national asset in the ensuing space programs."

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